The present invention relates to a method for determining the location, relative to the grab transporting it, of the end heads of rolls used for packing rolls of paper, cellulose, and board.
The invention also relates to an apparatus intended to apply the method.
The wide roll of paper coming from a paper machine is first of all transported to a slitter-winder and cut into rolls of suitable width. Next, the rolls are packed for transport. When paper rolls are packed, inner end heads are first of all placed on their ends, after which the necessary amount of wrapping is wrapped around the roll, the ends of which are folded on top of the inner end heads at the ends of the roll. An outer end head is glued on top of the folded wrapping and the internal end head, usually by hot-sealing. The inner end head is normally quite thick and protects the end of the roll from mechanical damage. The outer end head is, in turn, thinner and its task is to secure the package on the end of the roll and protect the roll from moisture. Often the color and patterning of the outer end head are used to give the roll a neat appearance. The length and diameter of the roll being packed are measured prior to packing and suitably sized end heads are selected for the ends of the roll of the basis of the measurement results.
End heads can be placed on the ends of rolls in many different ways. Placing the heads by hand is the oldest method, which is still very suitable for packing lines with a reasonably small capacity, or in applications in which there is no need to increase the level of automation. In this case, the packer simply places the inner heads by hand on the ends of the roll and correspondingly the outer heads onto heat-press plates, which press the outer heads onto the ends of the roll. The inner heads can also be moved manually with the aid of an air blast, or mechanically without touching them. The inner end heads are held on the end of the roll using a separate arm while the ends of the edges of the wrapping are being folded. The outer heads are, in turn, attached to the press plates by vacuum suction. In manual placing, the packer ensures that the heads of the right size are used on the roll and that they are correctly placed.
Various kinds of automatic end-head setting devices have been used already for a long time and many different kinds of them exist. A common feature of nearly all automatic head-setters is that there is a device including a grip for both ends of the roll, which transfers the head from a pile of heads onto the end of the roll. In one known end-head setter, there is a rotating arm located in a vertical guide, at the end of which is a rotating vacuum grip for gripping the heads. Such a head setter is usually used with separate head shelves located next to the setter. Heads are set on the end of the roll using this device, in such a way that the arm is moved along the vertical arm to the height of the shelf on which there are the correctly sized heads. The grab arm and the grab are rotated until the grab is parallel with the shelf top, after which the head is picked off the shelf and transferred to the end of the roll by rotating the arm and grab and moving them along the guide. In this type of device, there is usually no separate device for measuring the size and position of the end heads.
In another system, the end heads are placed in piles on the floor of the mill hall and are transferred to the end of the rolls using a portal-operation head setter. The transfer portal is built above the piles of heads and the head setters are generally installed on the same transversely movable guides. Thus, there must be a separate pile of heads of a specific size for each grab. U.S. Pat. No. 5,157,265 discloses a method for determining the size and position of end heads, which is suitable for use in connection with the above system. In this measurement method, the end head lifted by the grab is taken at a known speed past two pairs of photoelectric cells, so that the arrival of the front edge of the head at the position of the cells and the passing of the rear edge of the head are detected on the basis of the change in the signals of the photoelectric cells. The distance between the points of intersection can be calculated form the known speed of the head and the difference in the time of change in the signals. Because the shape of the head is known, its position and size can be determined. Because the lifting position of the grab relative to the head pile is known, the real position of the head pile can be determined from the position of the head on the grab.
Instead of the end-head handling methods described above, it is possible to use a standard model industrial robot with several degrees of freedom in head handling. Such a robot can be located in connection with a packing line, in such a way that it can be used to place an end head on each end of a roll. In order to make the robot operate efficiently, it must use a two-sided grab, by means of which it can pick up heads for both sides one after the other, by rotating the grab in between, thus eliminating the need for two lifting movements. Two robots can also be used, in which case a shorter stage time will be achieved.
The method disclosed in U.S. Pat. No. 5,157,265 has, however, several weaknesses, due to which it is not suitable for use in the transfer of end heads taking place with a robot. Because a two-side grab, in which the heads are on top of each other, must be used with the robot, the photoelectric cells are not able to distinguish from which edge of the head the signal changes, so that this method cannot be used when using a two-sided grab, unless depth-range detection is set for the photoelectric cells, so that they will detect only the desired head from heads lying on top of each other. Because only two photoelectric cells are used in the method, it cannot be used to detect edge damage. If a faulty part of the edge coincides with the path of the photoelectric cell, the size and position of the head will be calculated wrongly and the head may be taken to broke, even though in fact it might be completely usable. The rejection of a head is not in itself a problem, but after rejection a new head must be lifted, which of course will disturb the operation of the packing line. Usually, however, the head is taken to the press plate and the operator is given an error notification, when he will correct the position of the head visually or place a new head on the press plate. This can only be done when the movements of the robot are sufficiently slow while correction of error situations greatly disturbs the operation of the packing line. The movements of the robot must be slowed for reading of the position of the end head to be made with sufficient precision. Similarly, in an error situation, the movement must be stopped, so that the operator can safely enter the area of movement of the robot. Thus, it is possible to operate in the manner described above also when using robot head setting, but the greatest benefit will not, however, be gained from the robot, due to the necessarily slow speed of movement of the robot.
In addition, in this system the position of the head is compared with the position of the reference point of the grab, the location of which is known the whole time on the basis of the signal given by the movement sensors of the transfer apparatus. Thus, the method cannot be applied when using industrial robots, because with rapid movements the position data of the robot cannot be determined continuously. In the case of robot head setting, the reference point of the grab must be determined in some other way than from the position data of the robot, because the movement of the robot cannot be slowed/is not worth slowing during the measurement to such an extent that the position data can be read.
U.S. Pat. No. 5,376,805 discloses a method for determining the size and position of an end head relative to the grab transporting the head. The grab is moved by a robot with several degrees of freedom and the measurement of the head takes place in a separate measuring station. The measuring station has three photoelectric cells, passed which the head is taken. A detection element is fitted to the grab, with the aid of which the position of the tool-point of the grab is determined when the grab passes the first detection element. The position and size of the head is determined on the basis of the segments obtained with the aid of the signals given by the detectors from the edges of the head. In this solution too, a separate measuring station is used, so that the transfer of the head to the end of the roll is slow, or else a high-efficiency robot must be used, by means of which a high acceleration and deceleration can be achieved. If the size of the head is not determined, but is instead given as initial data to the system, only two photoelectric cells will be needed and two points for determining the position of the end head.